Method of magnetic resonance analysis employing cylindrical coordinates and an associated apparatus

ABSTRACT

A method of magnetic resonance imaging employs cylindrical coordinates and in one embodiment has an elongated catheter which is operatively associated with an RF pulse transmitting antenna and an RF pulse transmitting body coil. A main magnetic field is imposed on the region of interest. Circumferential phase encoding is accomplished by applying an initial RF pulse from either the catheter antenna or the body coil and subsequently applying an initial series of RF pulses with the source alternating between the antenna and the body coil. Radial phase encoding is effected by applying a first RF pulse which in a second embodiment is followed by a second RF pulse. A longitudinal gradient magnetic pulse is applied in the region of interest to spatially encode magnetic resonance signals. The cylindrical coordinate imaging is obtained by combining the circumferential phase encoding information the longitudinal magnetic encoding information with or without the radial phase encoding information. Other embodiments not employing catheter antennas employ two antennas with non-uniform phase profiles with the pulse sequences employed with the antennas. Corresponding apparatus is provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of magnetic resonance analysisemploying cylindrical coordinates and a catheter antenna and relatedapparatus and, more specifically, it relates to such magnetic resonanceimaging wherein alternating use of the catheter antenna and the bodycoil means as the source of the RF pulses facilitates obtaining highresolution images.

2. Description of the Prior Art

The advantageous use of non-invasive and non-destructive test procedureshas long been known in both medicine and industrial applications. Inrespect of medical uses wherein it is desirable to limit a patient'sexposure to potentially damaging x-ray radiation, it has been known toaccomplish imaging objectives through the use of other non-invasiveimaging procedures, such as, for example, ultrasound imaging andmagnetic resonance imaging. See, for example, U.S. Pat. Nos. 4,766,381;5,099,208; 5,352,979; and 5,512,825.

In a general sense, magnetic resonance imaging involves providing burstsof radio frequency (RF) energy to a region of interest of a specimenpositioned in a main magnetic field in order to induce responsiveemission of magnetic radiation from the hydrogen nuclei or other nuclei.The emitted signal may be detected in such a manner as to provideinformation as to the intensity of the response and the spatial originof the nuclei emitting the responsive magnetic signal. In general, theimaging may be performed in a slice or plane, or multiple planes, orthree-dimensional volume with information corresponding to theresponsively emitted magnetic radiation being delivered to a computerwhich stores the information in the form of numbers corresponding to theintensity of the signal. The computer may establish a pixel value as byemploying Fourier Transformations which convert the signal amplitude asa function of time to signal amplitude as a function of frequency. Thesignals may be stored in the computer and may be delivered with orwithout enhancement to a video screen display, such as a cathode-raytube, for example, wherein the image created by the computer output willbe presented through regions of contrasting black and white which varyin intensity or color presentations which vary in hue and intensity.

Obtaining ultra-high resolution in existing systems has been difficult.With conventional methods, microscopic or near-microscopic resolutioncan be achieved using high strength gradients with very short risetimes. However, these gradients are very expensive and the nervestimulation threshold imposes restrictions on maximum gradient strength.

SUMMARY OF THE INVENTION

The present invention has satisfied the above-described needs. In thepresent invention a high resolution is obtained without requiring theuse of high strength gradients. This is accomplished through the use ofunique cylindrical encoding methods and the associated apparatus.

The invention includes the use of a support member, a catheter antennaand an operatively associated body coil. In one embodiment whereinimaging of the blood vessels, such as in an investigation ofatherosclerotic plaques is employed a catheter antenna and body coil areoperatively associated with a catheter.

The method of the invention in one embodiment includes effectingmagnetic resonance imaging employing cylindrical coordinates asdistinguished from Cartesian coordinate systems. A main magnetic fieldis imposed in alignment with the catheter antenna on the region ofinterest of a specimen. Circumferential phase encoding is effected byapplying to the region of interest an initial RF pulse from the catheterantenna or the body coil means and subsequently applying an initialseries of RF pulses alternating between the catheter antenna and thebody coil means as the source of the RF pulses. Radial phase encoding iseffected by applying a first RF pulse from the catheter antenna. Alongitudinal gradient magnetic pulse is applied to the region ofinterest to spatially encode magnetic resonance signals. In oneembodiment, the cylindrical coordinate image is obtained by combiningthe information obtained from the circumferential phase encoding stepand the longitudinal frequency encoding step. In another, informationobtained from a radial phase encoding step is combined with theinformation obtained from the circumferential and longitudinal steps. Inone embodiment of the invention, the circumferential phase encoding isachieved through the use of an initial 90 degree RF pulse applied byeither the catheter antenna or the body coil means and a series ofalternating RF pulses originating from the catheter antenna or body coilmeans.

The radial phase encoding may be effected by amplitude modulationmethods. An initial RF pulse, which is a hard pulse, is provided. Asknown to those skilled in the art, a "hard pulse" is an RF pulse whichis non-adiabatic and non-slice/frequency selective. These pulsescommonly have a short duration which may be less than 1 msec.

In another embodiment immediately after the initial hard pulse, a 90degree RF pulse is applied to transform the amplitude modulation tophase modulation.

Depending upon the source of an RF pulse, the catheter antenna or thebody coil means, and the use in the circumferential phase encoding orradial phase encoding portions of the method, the pulses may beadiabatic or non-adiabatic.

The apparatus may have a support member and a catheter antenna and bodycoil means operatively associated therewith. Magnetic field generatingmeans for generating a main magnetic field on a region of interest areprovided. Circumferential phase encoding means are provided foralternately applying to the region of interest RF pulses from saidcatheter antenna and from the body coil means. Radial phase encodingmeans applied to the region of interest a first radial RF pulse from thecatheter antenna. Computer means for receiving data acquired from saidcircumferential phase encoding, said radial phase encoding, saidlongitudinal frequency encoding, and producing cylindrical image datatherefrom. The computer means also emit signals to control means forinitiating an RF pulse from said catheter antenna or said body coilmeans and to control means for generating gradient waveforms.

As employed herein, the reference to magnetic resonance analysisembraces both imaging and spectroscopy by producing chemical shiftspectra.

It is an object of the present invention to provide a method andapparatus for magnetic resonance analysis employing cylindricalcoordinates.

It is a further object of the present invention to provide such a methodand apparatus which employs a catheter antenna and facilitates highresolution imaging without requiring the use of high strength gradients.

It is another object of the present invention to provide such a methodand apparatus which may be employed with conventional magnetic resonanceimaging equipment.

It is a further object of the invention to provide such a system whichmay be employed in combination with a catheter so as to facilitateimaging or chemical shift spectra within an opening in a patient, suchas a blood vessel, and may be used for such purposes as atheroscleroticplaque investigation.

It is a further object of the invention to provide such a system whichis adapted for rapid and reliable functioning.

It is another object of the present invention to provide such a systemwherein spatially varying voxel sizes provide information.

It is a further object of the present invention to provide such a methodand apparatus which may be employed for 2-D or 3-D magnetic resonanceimaging in a cylindrical coordinate system.

These and other objects of the invention will be more fully understoodfrom the following description of the invention on reference to theillustrations appended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a magnetic resonance imagingsystem.

FIG. 2 is a schematic illustration of a catheter and loopless catheterantenna associated with the body coil and magnet of the presentinvention.

FIG. 3 is a schematic illustration showing the cylindrical coordinatesof the present invention.

FIG. 4a is a partially schematic illustration of a catheter of thepresent invention.

FIG. 4b is a fragmentary cross-sectional illustration of a portion of apatient's blood vessel with a catheter antenna positioned therein.

FIG. 5 is a block diagram of an embodiment of the apparatus of thepresent invention.

FIG. 6 is a flow diagram showing an embodiment of the method of thepresent invention.

FIG. 7 is a representation of pulses in an even embodiment ofcircumferential encoding pulses and the related gradient of the presentinvention.

FIG. 8 is a representation of pulses in an odd embodiment ofcircumferential encoding pulses and the related gradient of the presentinvention.

FIG. 9 is a representation of a radial amplitude modulation pulse andecho and the associated gradient.

FIG. 10 is a radial phase modulation pulse and subsequent pulse showingthe pulses, echo and related gradient.

FIG. 11 is an illustration of the pulses, even echoes and relatedgradient of the 3-D cylindrical encoding.

FIG. 12 is an illustration similar to FIG. 11, but showing odd echoes.

FIG. 13 illustrates the pulses, echoes and associated gradient for 3-Dcylindrical even encoding achieved with phase encoding in bothcircumferential radial directions.

FIG. 14 is similar to FIG. 13, but shows odd echoes.

FIG. 15 is a cross-sectional view of a biopsy needle embodiment of theinvention.

FIG. 16 is a schematic illustration of an endoscope employing thepresent invention.

FIG. 17 illustrates a phase map showing the phase sensitivity of acircular surface coil employable in the present invention.

FIG. 18 is a schematic illustration of a patient positioned partiallywithin apparatus of the present invention which employs a surface coil.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As employed herein, the term "patient" means members of the animalkingdom including human beings.

As used herein, the term "specimen" refers to any object placed in themain magnetic field for imaging and shall expressly include, but not belimited to patients, biological tissue samples, and test specimensremoved from such members of the animal kingdom. It shall includeinanimate objects which may be imaged by magnetic resonance or whichcontain water or sources of other sensitive nuclei.

As employed herein, the term "body coils" means any antenna thatgenerates a highly uniform RF magnetic field around the region ofinterest.

As employed herein, the term "catheter antenna" means any antenna thatgenerates an RF magnetic field that has a phase profilecircumferentially varying phase profile. Preferably, this antenna hasuniform phase along the longitudinal and radial directions.

FIG. 1 shows a schematic representation of the general concept ofmagnetic resonance analysis. An RF source 2 provides pulsed radiofrequency (RF) energy to the specimen which, in the form shown, is apatient 4 in the main magnetic field which is created by magnetic fieldgenerator 6. The specimen is generally aligned with the main magneticfield and the RF pulses are imposed perpendicular thereto. Where obliqueimaging is employed, the angle of impingement of the RF vectorrepresenting the spatial gradient in the magnetic field will beangularly offset from cylindrical coordinates. This arrangement resultsin excitation of the nuclei within the region of interest, which is thearea or volume to be imaged, and causes responsive emission of magneticenergy which is picked up by receiver 8.

The receiver 8 may be a catheter antenna which has a voltage induced init as a result of such responsive emissions of magnetic energy. Thesignal emerging from receiver 8 passes through analog-to-digital (A/D)converter 10 and enters computer 12. Within the computer 12 the FourierTransformations of signals convert the plot of amplitude versus time toa map of the distribution of frequencies by plotting amplitude versusfrequency. The Fourier Transformations are performed in order toestablish the intensity values and locations of specific pixels. Thesevalues may be stored, enhanced or otherwise processed and emerge to bedisplayed on a suitable screen, such as a cathode-ray tube 16, forexample.

As shown in FIG. 2, specimen 30, which in this case is a human being,has a head 31, a trunk 32, arms 33, 34, and legs 35, 36. The specimen 30is disposed adjacent to a body coil 37 within the main magnetic fieldgenerated by magnet 38. The magnet 38 may be any magnet suitable for usein a magnetic resonance imaging scanner, such as a permanent magnet, asuperconductor or a resistive magnet, for example. A catheter antenna 40which may be loopless catheter antenna has been introduced into thepatient 30 in a conventional manner through the femoral artery and intothe trunk 32 with the antenna being adjacent to the region of interest41. The loopless antenna may be of the type disclosed in U.S. patentapplication Ser. No. 08/638,934, the disclosure of which is expresslyincorporated herein by reference. The catheter 40 is operativelyassociated with magnetic resonance scanner 44. In a preferredembodiment, the catheter antenna is aligned with the main magnetic fieldB_(o). This is preferred, for example, where a loopless antenna isemployed as such an antenna's circumferential phase profile depends onthe orientation. The invention also may be employed with encodingmethods which will function for oblique orientation and will employantennas having minimum dependence on phase to orientation.

Referring to FIG. 3, the cylindrical coordinates will be considered. Thebody coil 60 is adapted to be in surrounding relationship with respectto the longitudinal axis of the catheter 62 which is the z axis.Considering r being a radial extent 64 and θ being the angle from the xaxis, the z axis will be going out of the page. The region of interest65 is also shown.

As shown in FIG. 4a, a catheter body 80 has a generally centrallydisposed electrically conductive portion 82 which is the catheterantenna projecting therefrom. A tuning/matching and decoupling circuit84 is provided within coaxial cable 86 and connector 88 is adapted toconnect the catheter to the control and processing means (not shown).

As shown in FIG. 4b, a patient's blood vessel 90 has blood (not shown)flowing in passageway 91 and plaque 92 on portions of the interior bloodvessel surface 93. The catheter 94 is a coaxial cable in the form shownand has antenna portion 95 projecting forwardly in the direction B_(o).In the portion of the catheter antenna 94 shown in cross-section, theelectrically conductive core 95 has a surrounding annular electricallyinsulative portion 96 and an outer conductive layer 97. Projectingportion 98 of core 95 is the most sensitive portion of the antenna 94and is positioned closely adjacent to the region of interest which, inthis context, is the placque 92. The placque 92 may be imaged in themanner to be described herein. The readout is along the longitudinalaxis of the catheter antenna 95. If desired, the catheter 94 or theprojecting antenna portion 98 thereof, or both, may be protectivelycovered by a thin layer of a resinous plastic material or other materialwhich will not interfere with the functioning of the system.

With respect to FIG. 5, the loopless catheter antenna 100 has its z axisprojecting wire adjacent to the body RF coil 102 and adjacent to themagnet 104 which generates the main magnetic field preferably, but notnecessarily in the z direction. The catheter antenna is the z axis forpurposes of this diagram. The RF amplifier 108 is interposed between thetransmit switch 110 and the body RF coil 102 and the transmit/receiveswitch 112. The transmit switch 120 responsive to computer 120establishes the operating mode of the catheter. By the computer 120controlling the position of the transmit switch 110, it is determinedwhether the RF pulse will be initiated by the body RF body coil 102 orthe catheter antenna 100. The transmit switch 110 is connected to thebody RF coil 102 by lead 106 and to the transmit receive switch 112 bylead 107. Regardless of which source of RF pulse is employed, thecatheter antenna 100 serves to receive the encoded magnetic resonancesignals from the specimen's region of interest. The output of thetransmit receive switch 112 passes through preamplifier 116 to receiver118 wherein the signals containing the acquired data is converted to aresponsive, related electrical signals which are delivered to computer120 over lead 122. The next cycle of operation is controlled by thecomputer 120 emitting a signal to the RF amplifier 108 over lead 121which, in turn, emits a responsive signal to the transmit switch overlead 123. The gradient amplifier 124 serves to supply the drive currentto the gradient coil 126 in cooperation with computer 120. The gradientcoil 126 provides a longitudinal gradient magnetic pulse along the zaxis to the region of interest to spatially encode magnetic resonancesignals. This is the longitudinal frequency encoding step.

Referring to FIG. 6, in the process of the present invention, thespecimen is placed within the magnetic field generated by the magnet140. The catheter is inserted into the target blood vessel 142 to thedesired position and the cylindrical encoding pulse sequence 144 isapplied. The data 146 is acquired by the computer 120. The acquired data146 is processed in the computer employing inverse FFT in order toproduce an image in the cylindrical coordinate system. The inverse FFTcould be 1-D, 2-D, 3-D, or 4-D, as desired. The image may be displayed152.

The image information generated within the computer may be displayed bysuitable means 130, such as a CRT, for example, or may be stored orproduced in hard copy as desired.

While we do not wish to be bound by their current understanding as tothe manner in which the present invention functions, it is presentedhere as a means of providing added information to those skilled in theart.

To achieve the circumferential Fourier encoding, controlled phase alongthe circumferential direction is required. In this invention, this isachieved by applying 90 and 180 RF pulses from the catheter antenna andbody coil in an alternating fashion. To explain the theory behind thismethod, it is desirable to show the effect of applying RF pulses fromdifferent body coils to the phase.

When the catheter antenna is aligned with the main magnetic field, thephase sensitivity varies in the circumferential direction. The receivedsignal with a catheter antenna is formulated as: ##EQU1## where M is thetransverse magnetization at the time of readout. The letters θ, r, and zrepresent the cylindrical coordinates. The letter j is the complexnumber √-1. The term exp(-jθ) represents the circumferential variationof the phase.

If a perfect 90 degree pulse is applied using a body coil with a phaseof φ, the transverse magnetization becomes:

    M=M.sub.z exp(jθ-jπ)                              (2)

where M_(z) is the longitudinal magnetization before the application ofthe RF pulse. Similarly, if a 90 RF pulse is applied using a catheterantenna, the magnetization becomes:

    M=M.sub.z exp(jφ+jθ-jπ)                       (3)

In this equation, the extra θ term compared to the body coil comes fromthe fact that the magnetic field produced by the catheter antenna variescircumferentially.

If a 180 RF pulse with a phase of φ is applied using a body coil, thetransverse magnetization before the application of the RF pulse becomesits conjugate with an additional phase as follows:

    M=M*.sub.-- exp(j2φ)                                   (4)

where M₋₋ is the transverse magnetization just before the RF pulseapplication. "*" represents the complex conjugation operation. If the180 RF pulse is applied using a catheter antenna, the magnetizationbecomes:

    M=M&.sub.-- exp(j2θ'j2φ)                         (5)

Using the information above and assuming there are no phase errors andno T2 decay, the transverse magnetization at each echo for the pulsesequence shown in FIG. 7 is calculated as:

    M.sub.1 =M.sub.z                                           (6)

    M.sub.2 =M.sub.z exp(j2θ)                            (7)

    M.sub.3 =M.sub.z exp(-j2θ)                           (8)

    M.sub.4 =M.sub.z exp(j4θ)                            (9)

    M.sub.5 =M.sub.z exp(-j4θ)                           (10) ##EQU2##

In FIGS. 7 through 14, the suffix "b" adjacent a number means that thespecific RF pulse was applied using the body coil means and the suffix"c" adjacent a number means that the RF pulse was applied by thecatheter antenna. The horizontal line adjacent to legend "RF" is a timeline with time increasing to the right. The echoes are shown as receivedon the time line. Underlying the RF pulses is G_(z), which is thegradient waveform that is employed in generation of a magnetic fieldgradient along the z direction. This magnetic field gradient controlsthe echo formation and frequency encodes along the z direction.

"Adiabatic" RF pulses are a special type of RF pulses that producesuniform flip angle even when there is variation in the applied powerlevel. This is especially useful for RF transmission from the surfacebody coils. See M. Garwood, K. Ugurbil, A. R. Rath, M. R. Bendall, S. L.Mitchell and H. Merkle, "Magnetic Resonance Imaging With AdiabaticPulses Using a Single Surface Coil for RF Transmission and SignalDetection," Magnetic Resonance in Medicine, 9(1):25-34, 1989. All of the90 degree and 180 degree RF pulses applied using the catheter antennaare adiabatic pulses.

In FIG. 7, the initial RF pulse 160 is a 90 degree RF pulse applied bythe catheter antenna. It is followed by a 180 degree RF pulse 162 fromthe body coil means. The next RF pulse 164 is a 180 degree pulse appliedby the catheter antenna. The pulses alternate as to source in thismanner. A cycle may have about 1 to 512 RF pulses, for example, and lastabout 5 msec to 1 sec. The cycle may be repeated every 10 msec to 10sec. during data acquisition. FIG. 7 illustrates the even k.sub.θ echoesof circumferential encoding.

FIG. 8 is similar to FIG. 7, except that it shows the odd k.sub.θ echoesof the circumferential encoding method. The first RF pulse is a bodycoil 90 degree RF pulse and the first 180 degree pulse is a catheterantenna pulse. For the pulse sequence shown in FIG. 8, the magnetizationat each echo can be calculated as:

    M.sub.1 =M.sub.z exp(jθ)                             (12)

    M.sub.2 =M.sub.z exp(-jθ)                            (13)

    M.sub.3 =M.sub.z exp(j3θ)                            (14)

    M.sub.4 =M.sub.z exp(-j3θ)                           (15)

    M.sub.5 =M.sub.z exp(j5θ)                            (16)

with a proper rearrangement of the data, one obtains: ##EQU3## If theeffect of the readout gradient is added, the following expression isobtained:

    M.sub.k.sbsb.θ.sub.k.sbsb.z =exp(j(k.sub.θ +1)θ)exp(jk.sub.z z)                                (18)

FIG. 7 illustrates the pulse sequence to acquire even number K-spacelines which correspond to even values of K.sub.θ in Equation 19.Substituting the above equation into Equation 1, the followingrelationship is obtained: ##EQU4##

An inverse 2-D FFT two dimensional Fast Fourier Transformation of theacquired data in the θ and z directions will result in an image in thecylindrical coordinate system with a radial projection.

Radial Fourier encoding is achieved in a similar manner to that ofrotational frame zeugmatography. See, D. I. Hoult, "Rotating FrameZeugmatography," Phllos Trans R. Soc. Land B. Biol. Sci., Vol 289, pp.543-7 (1980). A hard pulse is applied using the catheter antenna withincremental amplitude which modulates the amplitude of the magnetizationalong the radial direction (see FIG. 9). A "hard pulse" is a pulse ofshort duration which may, for example, be on the order of about 10 μsecto 1 msec from which there is no gradient. With radial amplitudemodulation, the amplitude of the RF pulses is increased at each phaseencoding step. The amount of amplitude increase depends on the desiredfield of view. The number of cycles employed depends on the desiredresolution. The RF pulses are non-adiabatic and are applied by thecatheter antenna and have a phase of 90 degrees. Examples of radial RFpulses are shown in FIGS. 9 and 10. Alternatively, a 90 degree RF pulseis applied just after the initial hard pulse to transform the amplitudemodulation to the phase modulation (see FIG. 10). The second pulsesequence will be analyzed. The analysis of the former sequence is verysimilar except the Fourier transformation is replaced with the sinetransformation. In the radial phase modulation, the amplitude of thefirst RF pulse is increased at each phase encoding step. The second RFpulse is an adiabatic pulse. The first RF pulse 170 has a zero phase andthe second pulse 172 has a phase of 90 degrees. Both pulses are appliedby the catheter antenna.

This method relies on the fact that when an RF pulse is applied using acatheter antenna, the flip angle varies radially with the function of1/r. For the pulse sequence shown in FIG. 10, the transversemagnetization after each RF pulse can be written as:

    M.sub.k.sbsb.r =M.sub.z exp(jk.sub.r β/r)exp(jθ)(20)

where β stands for the incremental amplitude of the RF pulse. By addingthe effect of readout gradient to the above equation, the followingresult is obtained:

    M.sub.k.sbsb.r.sub.k.sbsb.z =M.sub.z exp(jk.sub.r β/r)exp(jk.sub.z z)exp(jθ)                                           (21)

Substituting the above expression into Equation 1, the followingrelation is obtained: ##EQU5## An inverse 2-D FFT of the acquired dataover variables k_(r) and k_(z) will result in an image in the 1/r-zcoordinate system with a circumferential projection: ##EQU6## After 1/r³correction and doing (β/r) to r transformation, the r-z image isobtained. In this method, the voxel size increases with r³ and thesensitivity of the catheter antenna decreases by 1/r. Overall, thesignal to noise ratio of the images increases by r². A uniform signal tonoise ratio image can be obtained using an RF pulse that generates aflip angle inversely proportional to its amplitude.

By combining the radial and circumferential encoding methods, a 3-Dcylindrical encoding pulse sequence is obtained as shown in FIGS. 11 and12. The first pulse 176 in FIG. 11, which shows even k.sub.θ echoes of3-D cylindrical encoding, is a short hard pulse. The first pulse 178 inFIG. 12, which shows odd k.sub.θ echoes of 3-D cylindrical encoding isalso a short hard pulse. Radial encoding is achieved by the spinpreparation sequence. A crusher after the excitation from the catheterantenna kills the transverse component of the magnetization. Thelongitudinal magnetization becomes radially amplitude-modulated. Thecircumferential encoding is applied after this preparation period. Theanalysis of the pulse sequence is very similar to previous methodsdisclosed herein and will be apparent to those skilled in the art fromthe foregoing disclosure.

Another implementation of 3-D cylindrical encoding is given in FIGS. 13and 14 which respectively show even k.sub.θ and odd k.sub.θ echoes of3-D cylindrical encoding with phase encoding being achieved in bothradial and circumferential directions. In this sequence, bothcircumferential and radial encoding are done using phase modulation. Theeven echoes are obtained simply by replacing the adiabatic 90 degree RFpulse in FIG. 13 with a hard pulse, the amplitude of which is increasedat each radial encoding step (FIG. 13). To obtain odd echoes, however,several RF pulses are employed to achieve the proper encoding. Ananalysis of the first four RF pulses give the result of additionalradial encoding to the odd k.sub.θ lines.

The present invention provides a unique phase encoding method for 2-D or3-D magnetic resonance imaging in the cylindrical coordinate system. Themethod uses gradient magnetic pulses in only the readout or longitudinaldirection. Radial and circumferential encoding are done using RF bodycoils with the information obtained from these encoding steps with theinformation obtained from the longitudinal phase encoding. In oneembodiment, the information obtained from the phase encoding step iscombined with only the information obtained from the longitudinalfrequency encoding steps.

It will be appreciated that for some uses, such as internal blood vesselimaging, for example, it will be desirable for the catheter antenna tobe flexible, not all uses require such flexibility.

It will be appreciated that the present invention may be employed toimage stationary or moving objects.

While the present disclosure has focused on the preferred use of thepresent invention in connection with a catheter, the invention is not solimited. A suitable support member other than a catheter which providesone antenna in combination with another RF source which have non-uniformphase profiles can be employed with the pulse sequences. An example isshown in FIG. 15.

FIG. 15 is a schematic illustration of an embodiment of the presentinvention employed with a biopsy needle 220 which is composed of amaterial which is magnetic resonance compatible, such as a ceramicmaterial as distinguished from a steel material, for example. In thisembodiment the specimen 224 contains a lesion 226 from which a sample228 has been obtained by the biopsy needle 220. In this embodiment, theneedle coil 232 serves the function of the antenna in the priorembodiment. The needle coil 232 which is fixedly secured to the exteriorof the needle sheath 240 may be a 2 or 4 conductor needle coil havingthe general configuration in 2 or 4 conductor coil. A tuning andmatching circuit 244 is electrically connected to both the needle coil232 and preamplifier 246 which serves to amplify the signal before itenters the computer (not shown in this view) for further processing. Inthis embodiment, the needle coil 232 need not be flexible and theapparatus need not enter a natural passageway within the patient. Theneedle coil 232 may be secured to the needle by a suitable glue or resinor in the case of a ceramic material, by depositing the conductor ontothe ceramic by methods well known to those skilled in the art ofelectronic integrated circuit fabrication. The conductors are thensheathed with insulating material. In the pulse sequences of FIGS. 7-14,the biopsy needle can replace the catheter antenna and producecylindrically encoded images around the needle. The biopsy needle of thetype shown in United States patent application Ser. No. 08/457,833 filedJun. 1, 1995 may be employed in connection with the present invention.This application is expressly incorporated herein by reference.

Referring to FIG. 16, an embodiment of the invention employed in anendoscope will be considered. A patient 260 has an endoscope 264inserted through mouth 268 into the esophagus 270. The antenna 272 whichmay be made of coaxial cable has a sensitive end portion 274 which, inthe form shown, projects along the z axis. The region of interest orimaging volume 280 is shown as are the r and θ coordinates. The antenna272 is delivered to the esophagus by the endoscope 264 which serves as asupport surface therefor. In the pulse sequences of FIGS. 7 through 14,the endoscope can replace the catheter antenna and produce cylindricallyencoded images around the endoscope.

A phase map of circular surface body coil's phase sensitivity as used inanother embodiment of the invention which does not employ the catheterantenna is shown in FIG. 17. The radius of the coil is 1 unit andcontour lines are separated by 10 degrees. The x axis and y axisrepresent spatial coordinates with respect to the coil. In FIG. 17 a 1unit surface coil is placed at the origin with the axis of the coilbeing the y axis.

When the circular surface body coil is used, the two-dimensional Fouriertransform of the data will result in spectra of the voxels shown,separated by the contour lines in FIG. 17.

The method of achieving the circumferential encoding may be identical tothe one described herein in connection with cylindrical encoding using acatheter antenna with the surface coil replacing the catheter antenna.As shown in FIG. 18, a patient 300 is coaxial with the body coil 304.The axis A of the circular surface coil 308 will not be coaxial with thebody coil and preferably will be perpendicular thereto, although otherangular positions may be employed. A fast spin echo-like pulse sequenceis applied by alternating the source of the transmitter from the surfacebody coil to the body coil as shown in the pulse sequences shown inFIGS. 7 through 14. In this embodiment, the RF pulses that are appliedby the catheter antenna and the body coil in the other embodiment areapplied by the surface coil. With reference to FIG. 5, the surface coil308 is electrically connected to the scanner's transmit/receive switch312 and the body coil 304 is electrically connected to switch 110.

The present invention may be employed in spectroscopy to determine anddisplay chemical shift spectra by minor variation in the RF pulses in amanner well known to those skilled in the art.

Someone skilled in the art may convert the consequences shown in theFIGS. 7 through 14, for example, by removing the readout gradientwaveforms to magnetic resonance spectroscopic imaging techniques. Aswill be known to those skilled in the art, spectroscopic imaging may beachieved by employing other gradients such as crusher gradients, forexample, or no gradients. In this way, one can obtain high resolutionspectroscopic images of a specimen without requiring high strengthgradient waveforms.

It will be appreciated that the present invention provides an efficientmethod and apparatus for magnetic resonance imaging employingcylindrical coordinates. The invention may be employed advantageouslyfor catheter antenna imaging, as well as other embodiments. Alternatingtwo sources of the RF pulse generation is employed.

Whereas particular embodiments of the invention have been describedherein for purposes of illustration, it will be evident to those skilledin the art that numerous variations of the details may be made withoutdeparting from the invention as defined in the appended claims.

We claim:
 1. A method of magnetic resonance analysis of a specimenemploying cylindrical coordinates comprisingproviding a support memberhaving an RF pulse transmitting catheter antenna and RF pulsetransmitting body coil means operatively associated therewith, imposinga main magnetic field on a region of interest of said specimen,effecting circumferential phase encoding by applying to said region ofinterest an initial RF pulse from said catheter antenna or said bodycoil means and subsequently applying an initial series of RF pulsesalternating between said catheter antenna and said body coil means asthe source of said pulses, employing receiver means to receive magneticenergy received from said specimen responsive to said RF signals andemitting responsive magnetic resonance signals to processing means, andcreating in said processing means a cylindrical coordinate image orchemical shift spectra of said region of interest by using informationobtained during said circumferential phase encoding step.
 2. Themagnetic resonance imaging method of claim 1 includingemploying saidmethod in magnetic resonance imaging, and applying longitudinal gradientmagnetic pulses to said region of interest to spatially encode magneticresonance signal.
 3. The magnetic resonance imaging method of claim 2includingeffecting radial phase encoding by applying to said region ofinterest a first radial RF pulse from said antenna, and combining insaid processing means information obtained from said radial phaseencoding step with said information obtained from said circumferentialphase encoding step and said longitudinal encoding step.
 4. The magneticresonance imaging method of claim 2 includingemploying an elongatedcatheter as said support member.
 5. The magnetic resonance imagingmethod of claim 2 includingemploying as said initial RF pulse a 90degree pulse and employing as said subsequently applied series 180degree pulses.
 6. The magnetic resonance imaging method of claim 4includingemploying as said initial pulse an RF pulse from said catheterantenna.
 7. The magnetic resonance imaging method of claim 4includingemploying an RF pulse from said body coil means as said initialpulse.
 8. The magnetic resonance imaging method of claim 3includingafter said first radial RF pulse applying a second RF pulse byeither said catheter antenna or said body coil means.
 9. The magneticresonance imaging method of claim 4 includingemploying said method whilesaid elongated catheter is disposed in an opening in a patient's body.10. The magnetic resonance imaging method of claim 4 includingemployingsaid method when said elongated catheter is disposed in a patient'sblood vessel.
 11. The magnetic resonance imaging method of claim 10includingemploying said method in gathering information regardingatherosclerotic plaque.
 12. The magnetic resonance imaging method ofclaim 2 includingapplying 90 degree and 180 degree pulses of saidinitial series of RF pulses by said catheter antenna as adiabaticpulses.
 13. The magnetic resonance imaging method of claim 2includingapplying as said initial RF series of pulses about 1 to 512 RFpulses of 180 degrees.
 14. The magnetic resonance imaging method ofclaim 2 includingafter said initial series of RF pulses repeating saidinitial RF pulse and said initial series of RF pulses.
 15. The magneticresonance imaging method of claim 2 includingemploying an inverse 2-DFFT of the data acquired in the θ and z directions in saidcircumferential phase encoding step to create an image in thecylindrical coordinate system with a radial projection.
 16. The magneticresonance imaging method of claim 3 includingemploying an inverse 3-DFFT of the data acquired in the θ, r and z directions in saidcircumferential phase encoding step to create an image in thecylindrical coordinate system with a radial projection.
 17. The magneticresonance imaging method of claim 3 includingapplying said first radialRF pulse in said radial phase encoding as a non-adiabatic pulse.
 18. Themagnetic resonance imaging method of claim 17 includingapplying saidfirst RF pulse with a phase of 90 degree.
 19. The magnetic resonanceimaging method of claim 3 includingmodulating by said first radial RFpulse the amplitude of the magnetization along the radial direction byincreasing the amplitude of said first radial RF pulse at successivephase encoding steps.
 20. The magnetic resonance imaging method of claim3 includingapplying by said catheter antenna a second radial RF pulseafter said first radial RF pulse during said radial phase encoding totransform the amplitude modulation to phase modulation, and employing a90 degree RF pulse as said second radial RF pulse.
 21. The magneticresonance imaging method of claim 20 includingemploying an adiabaticpulse as said second radial RF pulse.
 22. The magnetic resonance imagingmethod of claim 2 includingemploying phase encoding in both saidcircumferential phase encoding and said radial phase encoding steps. 23.The magnetic resonance imaging method of claim 4 includingeffectingreadout of said longitudinal frequency encoding step along thelongitudinal axis of the cylindrical coordinates.
 24. The magneticresonance imaging method of claim 20 includingapplying said secondradial RF pulses by said catheter antenna means.
 25. The magneticresonance imaging method of claim 2 includingemploying as said catheterantenna a loopless catheter antenna.
 26. The magnetic resonance imagingmethod of claim 2 includingsaid support member being a biopsy needle.27. The magnetic resonance imaging method of claim 2 includingemployinggradients solely in the z direction.
 28. The magnetic resonance imagingmethod of claim 1 includingemploying an endoscope as said supportmember.
 29. The magnetic resonance imaging method of claim 28includingemploying said method to create said chemical shift spectra,and employing a gradient other than a readout gradient in said method.30. The magnetic resonance imaging method of claim 1 includingemployinga circular surface coil as said RF pulse transmitting antenna.
 31. Themagnetic resonance imaging method of claim 30 includingpositioning saidcircular surface coil with its axis not coaxial with the axis of saidcoil means.
 32. Apparatus for magnetic resonance of a specimen employingcylindrical coordinates comprisinga support member, a catheter antennaoperatively associated with said support member, body coil meansoperatively associated with said support member, magnetic fieldgenerating means for generating a main magnetic field in a region ofinterest of said specimen, circumferential phase encoding means foralternately applying to said region of interest RF pulses from saidcatheter antenna and from said body coil means, computer means forreceiving data acquired from said circumferential phase encoding andproducing a cylindrical coordinate image or chemical shift spectratherefrom, receiver means for receiving magnetic energy from saidspecimen responsive to said RF signals and emitting responsive signalsto said computer means.
 33. The magnetic resonance analysis apparatus ofclaim 32 includingsaid apparatus being imaging apparatus, and gradientgenerating means for establishing a longitudinal gradient in said regionof interest.
 34. The magnetic resonance imaging apparatus of claim 33includingradial phase encoding means for applying a first radial RFpulse from said antenna, and said computer means having means forcombining data from said radial phase encoding step with data from saidcircumferential phase encoding.
 35. The magnetic resonance imagingapparatus of claim 33 includingsaid support member being an elongatedcatheter.
 36. The magnetic resonance imaging apparatus of claim 33includingdisplay means for receiving cylindrical coordinate image datafrom said computer means and displaying the same.
 37. The magneticresonance imaging apparatus of claim 33 includingsaid control meanshaving means for alternately providing RF pulses from said body coilmeans and said catheter antenna means.
 38. The magnetic resonanceimaging apparatus of claim 33 includingsaid circumferential phaseencoding means having means for applying an initial RF pulse from saidcatheter antenna or said body coil means and subsequently applying aninitial series of RF pulses alternating between said catheter antennaand said body coil means as the source of said pulses.
 39. The magneticresonance imaging apparatus of claim 34 includingsaid radial phaseencoding means having means for applying a first radial RF pulse fromsaid catheter antenna.
 40. The magnetic resonance imaging apparatus ofclaim 38 includingsaid computer means having means for employing aninverse 2-D FFT of the acquired data in the θ and z directions to createsaid cylindrical coordinate image data.
 41. The magnetic resonanceimaging apparatus of claim 38 includingsaid computer means having meansfor employing an inverse 3-D FFT of the acquired data in the θ, r and zdirections to create said cylindrical coordinate image data.
 42. Themagnetic resonance imaging apparatus of claim 32 includingsaid supportmember being a biopsy needle.
 43. The magnetic resonance imagingapparatus of claim 33 includingsaid catheter antenna being a looplesscatheter antenna.
 44. The magnetic resonance imaging apparatus of claim33 includingsaid circumferential phase encoding means including meansfor applying said catheter antenna RF pulses as adiabatic pulses. 45.The magnetic resonance imaging apparatus of claim 32 includingsaidsupport member being structured to be introduced into a patient.
 46. Themagnetic resonance imaging apparatus of claim 32 includingsaid supportmember being an endoscope.
 47. The magnetic resonance imaging apparatusof claim 32 includingsaid support member being a catheter structured tobe introduced into a patient's blood vessel.
 48. The magnetic resonanceimaging apparatus of claim 32 includingsaid support member being anendoscope.
 49. The magnetic resonance imaging apparatus of claim 32includingsaid antenna being a circular surface coil.
 50. The magneticresonance imaging apparatus of claim 32 includingsaid antenna having anon-uniform phase profile from said body coil means.
 51. The magneticresonance imaging apparatus of claim 33 includingsaid gradientgenerating means having means for generating a gradient other than saidlongitudinal gradient.
 52. The magnetic resonance imaging apparatus ofclaim 51 includingsaid gradient being a crusher gradient.